Have you tried activating "inf items in battle" while fishing? If you have at least 2 bait, it shouldn't go down... I think (I know it works for Quarz, Accessories and Weapons... not sure if it works for fish though)
I have a humble request to add the ability to change the current difficulty of the game. The save files are compressed in some way, so it won't be straightforward to hex edit a value in them. There's gonna be a demand for a way to change it, I reckon.
For anybody who needs it, those are the additions you can put in the "Set items to 99" script
I just screwed a bit around with the values in the script, to see what's what (so I wouldn't know how to separate Guns from Event/Key Items or the Tonfas from the Fishing Items) CODE
cmp r8,15 // Books
je ingr
cmp r8,12 // Quarz
je ingr
cmp r8,16 // tools attak food
je ingr
cmp r8,A // Equipment Armor
je ingr
cmp r8,B // Equipment Shoes
je ingr
cmp r8,C // Equipment Accessories
je ingr
cmp r8,1 // Equipment Guns (Ellie) + Event!? +Fish
je ingr
cmp r8,2 // Equipment Staffs (Tio)
je ingr
cmp r8,3 // Equipment Axes (Randy)
je ingr
cmp r8,0 // Equipment Tonfas (Lloyd) + Fishing Tools&Bait
je ingr
Honestly I'd like pointers for things like U-Materials, Divine Cloths, and Zemurian Ores as well. Or just inventory editor pointers in general.
This isn't the exact thing you wanted but it's the best I can do ...
A version of Tuuuup!' s script that sets all event items, books, guns and fish you have in your inventory to 65 (after opening the inventory and looking at the tab)
Having too many Quest-Items COULD softlock your game, so be careful with that and have backup saves ready.
it seems the same in old zero no kiseki table, you can try to copy the mira address, and in pointer edit the value:
elie = 44d4+1bf5
tio = 44d4+1bf6
randy = 44d4+1bf7
then change the type to byte.